686 Dent J (2012) 23(6): 686-691 Braz
J.P. Lyra e Silva et al.
ISSN 0103-6440
Effect of Plasma Welding Parameters on the Flexural Strength of Ti-6Al-4V Alloy João Paulo LYRA E SILVA1 Alfredo Júlio FERNANDES NETO1 Luís Henrique Araújo RAPOSO2 Veridiana Resende NOVAIS2 Cleudmar Amaral de ARAUJO3 Luisa de Andrade Lima CAVALCANTE1 Paulo Cezar SIMAMOTO JÚNIOR4 1Department
of Occlusion, Fixed Prosthodontics and Dental Materials, Dental School, UFU - Universidade Federal de Uberlândia, Uberlândia, MG, Brazil 2Department of Operative Dentistry and Dental Materials, Dental School, UFU - Universidade Federal de Uberlândia, Uberlândia, MG, Brazil 3Mechanical Engineering School, UFU - Universidade Federal de Uberlândia, Uberlândia, MG, Brazil 4Health Technical School, ESTES, UFU - Universidade Federal de Uberlândia, Uberlândia, MG, Brazil
The aim of this study was to assess the effect of different plasma arc welding parameters on the flexural strength of titanium alloy beams (Ti-6Al-4V). Forty Ti-6Al-4V and 10 NiCr alloy beam specimens (40 mm long and 3.18 mm diameter) were prepared and divided into 5 groups (n=10). The titanium alloy beams for the control group were not sectioned or subjected to welding. Groups PL10, PL12, and PL14 contained titanium beams sectioned and welded at current 3 A for 10, 12 or 14 ms, respectively. Group NCB consisted of NiCr alloy beams welded using conventional torch brazing. After, the beams were subjected to a three-point bending test and the values obtained were analyzed to assess the flexural strength (MPa). Statistical analysis was carried out by one-way ANOVA and Tukey’s HSD test at 0.05 confidence level. Significant difference was verified among the evaluated groups (p0.05). The weld depth penetration was not significantly different among the plasma welded groups (p=0.05). Three representative specimens were randomly selected to be evaluated under scanning electron microcopy. The composition of the welded regions was analyzed by energy dispersive X-ray spectroscopy. This study provides an initial set of parameters supporting the use of plasma welding during fabrication of titanium alloy dental frameworks. Key Words: energy dispersive spectroscopy, flexural strength, plasma arc welding, scanning electron microscopy, titanium alloy.
INTRODUCTION The use of implants for prosthetic rehabilitation of edentulous patients is highly successful (1). However, a large part of the population remains without access to this treatment due to the inherent high costs. A considerable number of researchers attempted to optimize and simplify the original ad modum Brånemark implant rehabilitation with new techniques, alternative alloys and new welding methods, in an effort to expand the
availability of dental implants (2-4). Several studies have examined alternative prefabricated frameworks employing titanium and Ti-Al-V (Titanium-aluminumvanadium) bars welded to titanium abutments to simplify manufacturing, decreasing laboratory effort, and reducing costs and treatment time, while still maintaining good fit and biocompatibility (2-4). Titanium and its alloys are used for manufacturing dental prostheses because of their favorable biocompatibility, strength and modulus of elasticity.
Correspondence: Prof. João Paulo Lyra e Silva, Área de Oclusão, Prótese Fixa e Materiais Odontológicos, Faculdade de Odontologia, Universidade Federal de Uberlândia, Avenida Pará, 1720, Bloco 2B Sala 2B01, Campus Umuarama, 38401-136 Uberlândia, MG, Brasil. Tel: +55-34-3218-2222. Fax: +55-34-3218-2279. e-mail:
[email protected] Braz Dent J 23(6) 2012
Plasma welding on the titanium alloy beams
However, the melting temperatures of these metals are near 1,700ºC, requiring special procedures and equipment for casting, heat treatment and coating to prevent contamination (5). Although at room temperature titanium is covered by an oxide layer (TiO2) that provides corrosion resistance and biocompatibility, this metal is highly reactive at high temperatures and has a great affinity for hydrogen, nitrogen and oxygen. A reaction with these gases can decrease the properties of the material, so inert gas shielding is required during hightemperature processing (6,7). Manufacturing of dental prostheses usually requires permanent joining of metal components. Torch brazing welding is not effective in joining titanium or its alloys (8), and other welding techniques such as laser, TIG (tungsten inert gas) (9-11), and plasma methods (12) must be used. Plasma arc welding heats the material using a high-temperature ionized gas, and may be used to weld metals with excellent control and relatively low equipment cost (13). It produces joints superior to those obtained using torch brazing with alloys such as cobalt-chromium (CoCr) or nickel-chromium (NiCr) and also generates less distortion of the welded parts (9). Few studies have described technical standards for evaluating prosthetic frameworks produced using plasma welding. Thus, the aim of this study was to assess the effect of different plasma arc welding parameters on the flexural strength of titanium alloy beams (Ti-6Al4V) The hypothesis was that increased plasma welding periods would improve the flexural strength of Ti-6Al4V alloy beams.
MATERIAL AND METHODS Forty Ti-6Al-4V and 10 NiCr alloy beam specimens were prepared (40 mm long and 3.18 mm diameter) and divided into 5 groups (n=10). The titanium beams for the control group were not sectioned or subjected to welding. Groups PL10, PL12 and PL14 contained titanium beams sectioned and welded at a 3 A current for periods of 10, 12 or 14 ms, respectively. Group NCB consisted of NiCr alloy beams welded using conventional torch brazing. NiCr torch-brazed alloy beams and non-welded titanium bars served as negative and positive controls. The titanium specimens were prepared from grade 5 circular Ti-6Al-4V alloy bars 3.18 mm in diameter (⅛ in), as provided by the manufacturer (Realum, São Paulo, SP, Brazil). The bars of the control group were
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cut into 40-mm-long beams using a silicon carbide disk (Dentorium Products Co. Inc., Farmingdale, NY, USA) mounted in an electric motor (Kedel, Porto Alegre, RS, Brazil). The parts were cooled during cutting to minimize any possible distortion. Beams for groups PL10, PL12, and PL14 were cut into 20 mm pieces to provide a welding joint. The specimens for the NCB group were made using a silicone mold obtained from the prefabricated titanium bars and filled with autopolymerizing acrylic resin (Duralay; Reliance Dental Mfg Co., Worth, IL, USA) to obtain 20-mm-long and 3.18 mm diameter patterns, which were invested and next cast with NiCr alloy (Fit Cast-SB; Talladium do Brasil, Curitiba, PR, Brazil) using the lost wax technique (15,16). The prepared bars were ultrasonically cleaned in distilled water for 5 min (10). For plasma welding procedures, the titanium bars were held in a fixture to ensure uniformity of the welding position. The titanium specimens were welded using a plasma arc welder (Micromelt; EDG, São Carlos, SP, Brazil). The parts were connected to the positive terminal of the device using a clamp. The plasma gas was grade 4.5 argon (99.95% purity) and the argon atmosphere was attained prior to welding. After verifying if the specimen was correctly positioned, the plasma arc welding pulse was initiated by pressing a foot pedal. The argon flow continued for 2 s after welding to allow cooling in an inert atmosphere. For the NCB group, the NiCr bars were also placed in a metal matrix, to ensure correct positioning and uniformity of the welding position. The bars were then fixed and involved with autopolymerizing acrylic resin (Duralay; Reliance Dental Mfg. Co.). Then they were invested and heated at 750ºC in a furnace to remove the acrylic resin index. Torch-braze welding procedures were carried out using a 1.0 mm diameter torch (Draeger; Labor Dental, São Paulo, SP, Brazil) with butane-propane/oxygen gas mixture and a NiCrbased welding stick (Vera Solder; Aalba Dent Inc., Fairfield, CA, USA), which was used to join the NiCr alloy specimens. After the welding procedures were carried out, the welded regions of all specimens were checked radiographically with occlusal films (IO-41; Kodak Insight, Kodak, NY, USA) to detect any defect represented by radiolucent points in the joint using a x-ray unity (Spectro 70X; Dabi Atlante, RibeirãoPreto, SP, Brazil). The beams were tested for flexural strength in a Braz Dent J 23(6) 2012
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three-point bending design in a device equipped with two rods of 3 mm in diameter and span-length of 20 mm (15). A compressive force was applied by a 3-mm-diameter tip positioned at the center of the specimen at a 0.5 mm/min crosshead speed (11). The applied load was measured by a 500 N load cell attached to a mechanical testing machine (DL 2000; EMIC, São José dos Pinhais, PR, Brazil), adjusted to 4,500 N maximum load and collapse at 20%. The test was considered complete when fracture or permanent plastic deformation of the beam occurred, until a maximum 5 mm displacement. The flexural strength (MPa) was calculated using the following equation: fs = 8.F.L πD3
The non-welded area was subtracted from the total cross-sectional area of the specimens and the results were analyzed again using the previously mentioned statistical tests (α=0.05). Sequentially, three representative specimens fractured at the welded region for the different welding times (P10, P12 or P14) were randomly chosen to be evaluated under scanning electron microscopy (SEM). The specimens were mounted on aluminum stubs and the fractured surfaces were examined by SEM to check the welding penetration area and the characteristic of the failures (JSM 5600LV; JEOL, Tokyo, Japan). The composition of the welded regions was verified by energy dispersive X-ray spectroscopy (EDS).
RESULTS
where fs is the flexural strength in MPa, F is the determinant of fracture strength or elastic limit in N, L is the span-length, and D is the diameter of the beam in mm. The data were initially subjected to Shapiro Wilk and Kolmogorov-Smirnov tests for checking homoscedasticity. When homoscedasticity was confirmed, one-way ANOVA and Tukey’s HSD test for multiple comparisons of means were used (α=0.05). Data were analyzed using SPSS 15.0 statistical software package (SPSS Inc., Chicago, IL, USA). Afterwards, the plasma welded specimens were examined using stereomicroscopy at ×2.5 magnification (Leica MS5; Leica Microscopy Ltd., Switzerland) to determine the degree of weld penetration in the fractured region. Images were captured and analyzed using a digital camera (Moticam 2000; Motic Instruments Inc., Richmond, Canada) and a computer software (MoticImages Plus 2.0 ML; Motic Instruments Inc.) to obtain measurements of the welding depth penetration.
The mean flexural strength values and standard deviations for the tested groups are presented in Table 1. One-way ANOVA indicated significant difference among the groups (p0.05). The NCB group showed the lowest flexural strength values, although it was statistically similar to PL14 (Table 1). The welding depth penetration was not significantly different among the plasma welded groups (p=0.05) (Fig. 1). SEM analysis of the fractured surface revealed effective welding only on the periphery of the specimens, with bubbles forming failures along the welded area (Fig. 2A) and presence of dimples (cavities characterizing
Table 1. Flexural strength means (MPa) and standard deviations (SD) for the tested groups. Group
Mean (S.D)
Control
2,781.88 (38.86)a
PL10
1,552.02 (180.31)b
PL12
1,470.35 (105.56)b
PL14
1,354.34 (315.19)bc
NCB
1,145.35 (206.55)c
Different letters indicate significant difference among the groups in rows (p
